A method which uses a liquid refrigerant, such as liquid helium, retained in a cooling section has been commonly used.
However, in a conventional method using a liquid refrigerant, a time period during which a cryogenic temperature is maintained is several days at most, and the ultimate temperature is only 4.2 kelvins, due to, e.g., vaporization and diffusion of the liquid refrigerant. Additionally, cooling from room temperature is impossible without stopping rotation. In addition, maintenance, such as addition of the liquid refrigerant, requires great effort and cost.
There is also known a cryogen-free refrigerator as in, e.g., Patent Literature 1. A cryogen-free refrigerator is a mechanical refrigerator without a liquid refrigerant, and examples thereof include a GM refrigerator. Although use of a cryogen-free refrigerator eliminates the need for maintenance, such as replenishment of a liquid refrigerant, the use is problematic in that a power source and a helium gas pipe are connected to the cryogen-free refrigerator to be rotated.